1. Field of the Invention
The present invention concerns a method for correction of an image data set that has been acquired with a planar image detector, as well as a method for generation of an image from a raw image data set that was acquired with a planar image detector with two dynamic ranges.
2. Description of the Prior Art
An image that is acquired with an x-ray planar image detector without x-ray-absorbing subjects being located in the beam path (known as a light image) does not exhibit a homogeneous grayscale value distribution but instead shows a characteristic brightness curve. This is due to different causes that are based on specific properties of the detector pixels as well as curves based on the specific properties of the x-ray radiation generated by an x-ray radiator.
Even without exposure, every detector pixel also produces a specific dark current that is primarily temperature-dependent. Furthermore, each detector pixel exhibits a specific sensitivity such that different detector pixels supply different current values even given uniform radiation of x-rays. This specific sensitivity can be further influenced by the subsequent electronics with which the detector pixel is read.
In addition to the properties of the detector pixels, a further reason for the non-homogeneous grayscale value distribution arises from the x-ray radiation emanating from the x-ray tube, the intensity of which depends on the exit angle of the x-ray radiation from the x-ray tube. A vignetting of the x-ray image occurs due to the cone shape of the x-ray radiation; and the intensity of the x-ray radiation decreases toward the image border due to the Heel effect.
All of these factors contribute to a light image exhibiting a characteristic grayscale value distribution. This characteristic grayscale value distribution is reflected in an image data set of a subject that has been acquired and can lead to significant quality limitations of the image data set.
It is therefore necessary it calibrate the planar image detector such that the non-uniform grayscale value distribution that depends on the specific properties of the respective detector pixels is compensated. The calibration so attained is used to correct the image intensity values in the image data set of a subject that was acquired.
U.S. Pat. No. 5,506,880 and the corresponding DE 195 45 663 A1 disclose an x-ray system and a method with which radiographic real-time images (RTR images or “real time radiographic images”) can be corrected for inhomogeneities of the x-rays. Calibration samples are used for this purpose in order to measure a field of attenuation information for the various calibration samples.
DE 101 49 404 A1 discloses a method for correction of different transduction characteristics in the processing of the signals from image sensors arranged distributed in a plane, as well as an x-ray detector that can execute such a method.
GB 2 314 227 A likewise describes a method for calibration of a pixel-based imaging apparatus for consideration of non-linear response properties of a pixel element and/or differences between pixel elements.
A method for calibration of a planar image detector with two different dynamic ranges is described in the publication by Roos et al., “Multiple-gain-ranging readout method to extend the dynamic range of amorphous silicon flat-panel imagers”, Proc. SPIE Vol. 5368, pages 139 through 149, Medical Imaging 2004. Such planar image detectors typically have a high-sensitivity dynamic range and a low-sensitivity dynamic range. The high-sensitivity dynamic range supplies usable signals even at lower energy of the incident radiation, but is rapidly saturated with increasing incident radiation energy. The low-sensitivity dynamic range is saturated only at significantly higher radiation energy, however supplies noisy and therewith unusable signals at low radiation energy.
The dynamic ranges are individually used for an acquisition in what is known as “Fix Gain Mode”. Depending on the incident radiation energy to be expected, the matching dynamic range can be used. For many applications in radiology the use of an individual dynamic range is sufficient in order to generate qualitatively high-grade images. There are applications, however, such as digital subtraction angiography and the application of what is known as the “cone beam technique” in computed tomography, in which a larger dynamic range of the planar image detector is necessary in order to deliver qualitatively high-grade exposures. In operation known as “dual gain mode”, both of the aforementioned dynamic ranges are combined with one another in order to cover a larger dynamic range given an acquisition. Two image data sets that were each acquired in one dynamic range are combined in order to obtain an image of the acquired subject.
The calibration of such a planar image detector requires a certain technical effort. The method described in the article by Roos et al., Proc. SPIE Vol. 5368, pages 139 through 149, Medical Imaging 2004 essentially assumes that a calibration image is separately acquired for both dynamic ranges given a respective radiation dose suitable for a dynamic range. These two calibration images characterize the characteristic grayscale value distribution of the planar image detector for the dynamic range. The image data set of the corresponding dynamic range is then corrected using a calibration image. A further calibration image is also additionally acquired in the low-sensitivity dynamic range at the radiation dose at which the calibration image was acquired for the high-sensitivity dynamic range. The two dynamic ranges can be combined by comparison of these two calibration images since it is now known how the image intensity values of two image data sets that were acquired at the same radiation energy with the two dynamic ranges relate to one another.
When a calibration was implemented with the method described in Proc. SPIE Vol. 5368, pages 139 through 149, Medical Imaging 2004, however, for the most part vertically-arranged striped structures still remain in the image. These structures are all the more pronounced the more significantly that the radiation energy in the image acquisition deviates from the radiation energy that was acquired in calibration images. This is to be attributed to the fact that the detector pixels are incorporated into the planar image detector in columns and that the detector pixels are read out grouped in columns by the readout electronic. The detector pixels arranged in columns additionally often stem from different production charges and therefore additionally exhibit a slightly-different dependency of their sensitivity on the radiation energy. If the calibration was implemented at a single radiation dose and the image of this was acquired at a radiation energy range deviating from said single radiation dose, the calibration may be insufficient. This is reflected in detectable striped structures in the corrected image.